Microwave scattering as a plasma diagnostic in a Penning discharge

A microwave scattering apparatus consisting of 32.175-GHz Gunn diode in a conventional homodyne mixer configuation has been built to investigate plasma turbulence in a steady-state classical Penning discharge. The microwaves are incident on the plasma in the ordinary mode at a power level of 25 mW. The scattered microwave power is observed for scattering angles from 20 to 160° in a plane normal to the magnetic field axis. The scattered microwaves are fed to a balanced mixer where they are detected with crystal detectors. The detected signal is then displayed on a spectrum analyzer or fed into an analog-to-digital data handling system for analysis and correlation with other signals.

In these experiments the electron number densities ranged from 10⁸ to 10¹⁰/cm³ the magnetic field ranged from 0.1 to 0.4 T, the discharge current ranged up to 0.2 A, and the input power ranged from 50 to 400 W. The frequency spectrum, angular dependence, and scattering amplitudes were measured with the microwave scattering diagnostic. Data from the microwave scattering diagnostic were correlated with plasma operating conditions and with data from capacitive probes which measured electrostatic turbulence. The data indicate strong, low-frequency electrostatic turbulence in the plasma, caused by E × B drift.